This invention relates to a sensor for sensing tensile and compressive forces such as are applied to the hitch linkages of a tractor hitch.
A known draft force sensor is described in German Patent publication No. DE 3,515,126 published on Oct. 30, 1986. This sensor includes a hollow bendable tube with a fixed end and a free end. Mounted inside the tube is a rigid gauge rod which has a fixed end fixed to the free end of the tube. The gauge rod extends to a free end near the fixed end of the tube. A magnetic deflection sensor senses deflection of the rod free end. The hollow bendable tube has a substantially uniform outer diameter. With the application of a force the flexible body or tube deflects by an amount which depends upon the force applied. The force is applied to the free end of the tube near where the gauge rod is connected to the flexible tube so that the free end of the gauge rod will deflect to a new position.
With a bendable tube which has a substantially constant outer diameter along its length, the deflection will increase steadily over the entire length and the axis of the flexible body in the region of force application is deflected through a certain angle from its position of rest. This angle is transmitted to the gauge rod which is not subject to any bending forces. In addition, the gauge rod is given a translational movement in the force direction perpendicular to the axis of the flexible body by reason of the deflection of the flexible body. The change in position resulting from the angle of deflection and the translational movement are transmitted to the free end of the gauge rod, which assumes a new position as a result. It has been shown, however, that these two effects upon the position at the free end of the gauge rod have opposite results, so that a movement of the free end by reason of the inclination is partly offset by the translational movement, which reduces the sensitivity of the force sensor. This compensating effect is particularly strong in short flexible bodies.
The flexible tube must be so designed that it can absorb the maximum applied load elastically without deforming permanently under load. In addition, the high bending moment at the fixed end connection and the bearing pressure under the applied force must be considered in the design.
In a number of applications, static loads as well as dynamic loads must be considered. For example, in an agricultural tractor hitch control system, the drawbar pull between the tractor and soil working implement must be measured by force sensors, and the control system moves the hitch as a function of the measured force. The load on a force sensor may be subject to sharp variations due to uneven ground, rocks, etc., which may lead to shock and vibration.
In order to be able to transmit large forces, it is an advantage to design the flexible body as short as possible. In addition, a lengthening of the gauge rod, which could lead to a greater measurement signal, is possible only to a limited extent, since its bending due to its own weight coupled with a reduced frequency will impair the reproducibility of the measurements. Such a lengthened rod could be excited to resonant vibration by the least shock, thereby inducing deflections that could far exceed the deflections for the expected measurement.
The requirement for short flexible bodies and gauge rods leads to the result that the deflections at the free end of the gauge rod from its position of rest, and hence the measurement signal, are relatively insensitive to the applied force.